$10rt - 2s + 7t + 3 = 4s + 4t + 6$ Solve for $r$.
Combine constant terms on the right. $10rt - 2s + 7t + {3} = 4s + 4t + {6}$ $10rt - 2s + 7t = 4s + 4t + {3}$ Combine $t$ terms on the right. $10rt - 2s + {7t} = 4s + {4t} + 3$ $10rt - 2s = 4s - {3t} + 3$ Combine $s$ terms on the right. $10rt - {2s} = {4s} - 3t + 3$ $10rt = {6s} - 3t + 3$ Isolate $r$ ${10}r{t} = 6s - 3t + 3$ $r = \dfrac{ 6s - 3t + 3 }{ {10t} }$